The following disclosure relates to electrical circuits and signal processing.
A receiver in a communications system (hereafter referred to as a receiver) is typically designed to receive signals of varying strength. A wireless receiver (e.g., an IEEE 802.11, 802.11a, 802.11b, 802.11e, 802.11g, 802.11h, 802.11i, 802.11n, and/or 802.16 compliant receiver), for example, can receive weak signals from transmitters that are far away from the wireless receiver or strong signals from transmitters that are close to the wireless receiver. To adjust for differing signal strengths, a conventional wireless receiver uses one or more gain stages to amplify or attenuate the received signal. In a conventional wireless receiver that mixes the received signal down to an intermediate frequency (IF) signal and later mixes the IF signal down to a baseband signal, both the gain of the IF signal and the gain of the baseband signal typically are adjusted.
Direct-current (DC) offset voltages can be generated in many of the circuits in a receiver. DC offset voltages can be amplified in gain stages along with the received signal if the design of the receiver allows the DC offset voltages to enter the gain stages. When two circuits are coupled using alternating-current (AC) coupling (e.g., by using a series capacitor to connect the output of one circuit to the input of the other circuit), low-frequency signal components, including any DC offset voltage, are heavily attenuated.
The amplifiers in an IF gain stage of a wireless receiver can be AC coupled to one another because the IF signal does not include any low-frequency signal components of interest. The amplifiers in a baseband gain stage, however, are typically direct-current (DC) coupled, because the baseband signal can have low-frequency signal components of interest. A DC offset voltage generated by a circuit early in a DC-coupled baseband region of the wireless receiver will be amplified in the baseband gain stage. Conventional baseband gain stages can supply over 45 dB of gain, so a DC offset voltage can be amplified by a factor of 200 or more. The large amplification of a DC offset voltage can cause amplifiers in the baseband gain stage to run out of voltage range for a given supply voltage, hence distorting the received signal.
Both wireless receivers and wireline receivers (e.g., gigabit Ethernet or fiber-channel receivers) typically have DC-coupled baseband gain stages in which important signal filtering and amplification is performed. DC offset voltages are often present in baseband circuits because of, for example, input device mismatch, current source mismatch, or load mismatch. In some conventional receiver designs, a highpass filter is added to the baseband gain stage to filter out any DC offset voltage. High-order highpass filters are typically used so that as little as possible of the received signal is filtered out.
When a highpass filter is used to remove a DC offset from a signal, several problems can arise. One problem occurs when the frequency of an oscillator in a wireless receiver differs slightly from the frequency that a wireless transmitter oscillator provides. When the oscillator signal generated by the oscillator in the wireless receiver is used to mix a received signal down to baseband, the frequency difference between the transmitter and receiver oscillators will cause the frequency components of the baseband signal to be shifted, resulting in a frequency-shifted baseband signal. When a frequency-shifted baseband signal is filtered with the highpass filter, desirable frequency components may be attenuated. The attenuation of frequency components when there is a frequency difference between the transmitter and receiver oscillators can cause signal distortion and degrade the quality of the received signal. High-order filters typically must be used to minimize the amount of signal distortion and quality degradation.
Additionally, if a highpass filter is used to remove a DC offset in the baseband gain stage, the corner frequency of the highpass filter typically depends on the gain setting of the baseband gain stage. If the corner frequency depends on the gain setting, the frequency components that are attenuated in the received signal will vary as a function of received signal power. Variation in the frequency components that are attenuated is typically an undesirable characteristic for a wireless receiver. In addition, a tradeoff typically exists between how high the corner frequency is and the speed at which the received signal can be filtered. Faster filtering requires attenuating more of the received signal.
For certain communications standards, a baseband signal near DC is important, so highpass filtering of the signal is not ideal. For example, highpass filtering of the received signal can cause DC-wandering problems, where the receiver loses a reference that was provided by the DC component in the received signal.
For certain wireless communications standards, a low-IF wireless receiver can mix the desired signal to a low intermediate frequency instead of to baseband. In this situation, if other problems associated with low-IF receivers are resolved, the DC-offset problem can be alleviated because the signal can be processed without the DC component. However, this only works well for some communications standards. High-bandwidth standards, for example, can be difficult to implement using a low-IF wireless receiver because high-order filters typically are required to isolate channels in the received signal. Care must also be taken with low-IF wireless receivers to avoid image-folding problems.
FIG. 1 shows a conventional differential amplifier 200. A first transistor 210 and a second transistor 220 are biased into the active region by a biasing current 230. A differential input signal 240 is applied between the base of first transistor 210 and the base of second transistor 220. A differential output signal 250 is present between the collector of first transistor 210 and the collector of second transistor 220.
If first transistor 210 and second transistor 220 are perfectly matched transistors, a first resistor 260 and a second resistor 270 are perfectly matched resistors, and there is no DC component in differential input signal 240, there will be no DC component in differential output signal 250. If, however, first transistor 210 and second transistor 220 are not perfectly matched transistors, or if first resistor 260 and second resistor 270 are not perfectly matched resistors, a DC component will be present in differential output signal 250 when there is no DC component in differential input signal 240.
DC components generated by component mismatches anywhere in differential amplifier 200 have the same effect on differential output signal 250 as a properly chosen representative DC offset voltage 280 has on differential output signal 250 when the components of differential amplifier 200 are perfectly matched. All DC offsets generated in differential amplifier 200 can be represented by DC offset voltage 280. For example, a DC component in differential output signal 250 due to a mismatch between first transistor 210 and second transistor 220 can be modeled as being caused by representative DC offset voltage 280 applied to the base of first transistor 210 with first transistor 210 and second transistor 220 being perfectly matched. Likewise, a DC component in differential output signal 250 due to a mismatch between first resistor 260 and second resistor 270 can be modeled as being caused by representative DC offset voltage 280 applied to the base of first transistor 210 with first resistor 260 and second resistor 270 being perfectly matched.